The present invention relates to a method for producing a composite material which contains a support material and an ionic liquid, as well as a composite material and its use as synthetic catalyst.
Materials that consist of a solid support component and a liquid component immobilized thereon have been examined intensively in the more recent past. The supporting of the liquid component and substances possibly dissolved or suspended therein gives the composite excellent new properties. Above all, the immobilization of ionic liquids (IL) on porous support materials is the focus of the interest here. The resultant composites can be used in industrially important fields such as catalysis, gas cleaning, the purification of fuel mixtures, the separation of mixtures, rheology and many more.
In the field of catalysis, two thematically related concepts are studied above all:
In the case of the so-called SILP (Supported Ionic Liquid Phase) concept [J. Joni, M. Haumann, P. Wasserscheid, Advanced Synthesis and Catalysis 2009, 351, 423-431; J. Baudoux, K. Perrigaud, P.-J. Madec, A.-C. Gaumont, I. Dez, Green Chemistry 2007, 9, 1346-1351; A. Riisager, R. Fehrmann, M. Haumann, P. Wasserscheid, Topics in Catalysis 2006, 40, 91-101] (also called SILCA [P. Virtanen, H. Karhu, G. Toth, K. Kordas, J.-P. Mikkola, Journal of Catalysis 2009, 263, 209-219; J.-P. Mikkola, J. Wärna, P. Vitanen, T. Salmi, Industrial & Engineering Chemistry Research 2007, 46, 3932-3940] or SILC [H. Hagiwara, K.-H. Ko, T. Hoshi, T. Suzuki, Chemical Communications 2007, 2838-2840]), ionic catalyst solutions are immobilized on porous support materials. The ionic catalyst solution consists of at least one ionic liquid, as well as at least one further catalytically active component. The catalytically active component can be an organometallic complex compound, metal nanoparticle, an organic catalyst or also a biocatalyst such as e.g. an enzyme. In addition, the ionic liquid itself can also act as catalyst for a reaction or serve as co-catalyst for the dissolved or suspended catalyst.
In the case of the so-called SCILL (Solid Catalysts with Ionic Liquid Layer) concept, ionic liquids or compositions containing an ionic liquid are immobilized on a solid (pre-formed) catalyst. The properties of the heterogeneous catalyst change as a result. In some cases, a dramatically increased selectivity in favour of the desired product was able to be observed while activity remained constant [J. Arras, M. Steffan, Y. Shayeghi, D. Ruppert, P. Claus, Green Chemistry 2009, 11, 716-723; U. Kernchen, B. Etzold, W. Korth, A. Jess, Chemical Engineering & Technology 2007, 30, 985-994].
Both of the named concepts can be used in all known reactor designs, such as for example an aerated or unaerated suspension reactor, a bubble column reactor, a fluid-bed reactor or a fixed-bed reactor. The particular properties of ionic liquids, such as non-vaporability, make SILP and SCILL catalysts particularly suitable for continuous gas-phase processes in fixed-bed reactors.
As a rule, such catalyst compositions are produced according to the state of the art by wet-chemical impregnation. The ionic liquid and any further (catalytically active) components such as a homogeneous catalyst or metal nanoparticle are dissolved in a suitable solvent (or suspended or emulsified) and the support is then added.
In order to obtain a uniform coating of the support with the ionic catalyst solution, in the methods of the state of the art the quantity of solvent is greater than the pore volume of the support material used. The solvent of the resultant suspension is then removed slowly. An externally dry material with a visually uniform coating is thus obtained. The disadvantage of the method is above all the great length of time required. However, if the solvent vaporizes too quickly, the dissolved components precipitate prematurely and a poor coating results. The removal of the solvent has also been carried out by standing the suspension in air, by expulsion using a gas stream or by freeze-drying, but the length of time required is even greater in all these methods. The slow evaporation of the solvent is important however: because the quantity of solvent is greater than the pore volume of the support, not all of the ionic liquid used is located in the pores of the support. If evaporation is too fast, the ionic liquid precipitates in an uncontrolled manner and a uniform coating is not achieved. In order to achieve a uniform coating with this method, the length of time required is thus very great.
If the quantity of solvent is reduced, so that it is smaller than or equal to the pore volume of the support material, the term incipient wetness impregnation is used. The length of time required in this case is much less, but the coating is often non-homogeneous and not reproducible.
Thus, for example, WO 2006/122563 A1 discloses the production of an SILP catalyst in which a silicate support is stirred in a solution containing an ionic liquid, wherein the SILP catalyst is obtained after the solvent has been drawn off.
US 2005/0033102 likewise discloses the production of a supported ionic liquid, wherein a support is introduced into an ionic liquid.
In the same way as in WO 2006/122563 A1, WO 02/098560 discloses a method for producing a supported composition in which an ionic liquid is applied to a support by introducing a support into an ionic liquid dissolved in a solvent, followed by vaporization of the solvent. An immobilized ionic liquid is produced in the same way in WO 01/32308.
EP 1 364 936 B1 discloses the production of a supported ionic liquid by mixing an ionic liquid with a support.
In addition to the disadvantage of the great length of time required and the fact that the coating is often produced non-homogeneous and not reproducible, the methods according to the state of the art give satisfactory results only when pulverulent support materials are coated. In the case of shaped bodies such as tablets, spheres, cones, rings, strands, hollow strands, trilobes, solid cylinders, hollow cylinders or grit, a homogeneous distribution of the ionic catalyst solution on the support material cannot be readily achieved, i.e. the distribution of the ionic catalyst solution cannot be set specifically, and is random. This is to be attributed in particular to the properties of ionic liquids, such as for example strong surface tension and high viscosity compared with the solvent. The smaller the support particles to be coated are, the smaller the part played by aggregation effects due to van der Waals forces. In the case of powders, therefore, a homogeneous distribution of the coating can be achieved. In the case of more complex shaped bodies, however, the named properties of ionic liquids come to bear more strongly, with the result that a high uniformity of the coating cannot be achieved with the conventional methods.